1. Field of the Invention
The present invention relates to a plastic externally coated steel pipe which can be used at a wide temperature range of from -30.degree. C. to 120.degree. C.
2. Description of the Prior Art
Polyethylene coated steel pipes are now widely used as a plastic externally coated steel pipe. Since polyethylene is inexpensive, is chemically stable, and has excellent mechanical properties (impact resistance, flexibility, etc.), polyethylene is considered an excellent material for coating a steel pipe. However, since polyethylene has a low softening point, it is generally considered to be unsuitable for use at temperatures exceeding 80.degree. C. Recently, the high pressure operation of pipelines and the transportation of heavy crude oil have created a demand for a plastic coated steel pipe which will withstand higher temperatures, and which can be substituted for conventional polyethylene coated steel pipes. Also, in sub-temperature zones, laying operations are carried out even at temperatures around -30.degree. C. Accordingly, it is essential that the plastic coated steel pipes should also have an excellent impact resistance and flexibility at such low temperatures.
Since polypropylene has a higher softening temperature than polyethylene and is chemically stable, it is expected as a coating material which can be utilized for coated steel pipes used at a high temperature. Conventional polypropylene is, however, inferior to polyethylene in mechanical properties at a low temperature, and its application to steel pipes is very limited.
The present inventors have already discovered that polypropylene consisting of a block copolymer of propylene and ethylene, in which the ethylene content is adjusted to an adequate range, exhibits improved mechanical properties at a low temperature while a high softening temperature is retained (See: Tsuchiya et al: Papers presented at the 4th International Conference on the Internal and External Protection of Pipes, p 307, 1981). The present inventors have made further investigations into the application of such a polypropylene to a steel pipe. As a result, the present inventors have discovered that an excellent adhesive strength is attained in the coated steel pipe only when a modified polyolefin adhesive is used and that, when an impact according to a procedure described in ASTM G14 is applied to the coating layer of a coated steel pipe obtained through the use of a conventional modified polyolefin adhesive, cracks are generated in the coating layer at a temperature of -20.degree. C. or less, and thus the impact resistance at -30.degree. C. required in a sub-temperature zone cannot be attained. Furthermore, it was discovered that, even if a pretreatment is applied to the steel pipe to form a conventional epoxy primer layer thereon, when a cathode potential is applied to the steel pipe for the purpose of preventing electrical corrosion, the disbondment (or delamination) of the coating layer (hereinafter abbreviated as "cathodic disbondment") becomes gradually more conspicuous with each increase in temperature.
Furthermore, even when polypropylene coated pipes are used at high temperatures under the ground, through-flaws sometimes occur due to the penetrating action of the surrounding gravel. Accordingly, the depth of penetration at high temperatures is preferably as little as possible. However, plastic coated steel pipes satisfying the above-mentioned requirements have not been developed.